Process of coating a refractory body with boron nitride and then reacting with aluminum



United States Patent Ofiice 3,084,060 Patented Apr. 2, 1963 PROCESS GF(IOATING A REFRACTORY BODY WliTH BURON NITRIDE AND THEN REAC'HNG WITHALUMINUM Charles A. Baer, Richardson, Ten, and Philip J. Clough,

Reading, and Robert W. Steeves, Nahant, Mass, assignors, by mesneassignments, to National Research Corporation, Cambridge, Mass., acorporation of Massachusetts No Brewing. Filed Apr. 25, 1%0, Ser. No.24,238

2 Claims. ((31. Ill-5.1)

This invention relates to coating and more particularly to the coatingof various substrates with aluminum. This application is in part acontinuation of the copending 1 application of Baer et 211., Serial No.786,675, filed January 14, 1959, now abandoned.

In the vacuum evaporation of aluminum where aluminum is heated to anelevated temperature on the order of 1200 to 1300 C., or above, one ofthe principal technical problems to be solved has been to find acrucible which is resistant to attack by the high-temperature aluminum.The same problem is encountered when a portion of thealuminum-evaporating source which is at elevated temperatures is exposedto a high density of aluminum vapors. This situation arises in thosecases Where it is desired to deflect or concentrate the flowing aluminumvapors to provide, for example, lateral or downward evaporation.

Some progress has been made in providing aluminumresistant structures,but these structures have not always been adequately simple to constructor cheap to maintain. While some progress has been made, greatdifliculty has been experienced in providing a completely satisfactorymethod for producing such a source, particularly one which is of acomplex geometric shape. Equally, the art has only with difficulty beenable to produce a stream of aluminum vapors which is directed laterallyor downwardly from the source. Such a stream is particularly useful whendiscrete objects such as nuts and bolts and powders are to be coated, orwhen two sides of a continuous substrate are to be coated at the sametime.

Accordingly, it is a principal object of the present invention toprovide a method for producing a source of aluminum vapors which canhave wide latitude of design and still be reasonably simple andinexpensive to construct.

Still another object of the invention is to provide a source of aluminumvapors of the type described above which is simple to manufacture anduse.

Another object of the invention is. to provide a source of the abovetype which can be produced from relatively inexpensive materials.

Still another object of the invention is to provide a means forconfining high-temperature aluminum Vapors to produce a stream ofaluminum vapors much more concentrated than can be produced bypreviously-known techniques.

Still another object of the invention is to provide a mechanism forproviding a high intensity stream of aluminum vapors capable ofproviding rapid coating of a discrete object with minimum transfer ofheat to the object.

Other objects of the invention will in part be obvious and will in partappear hereinafter.

The invention accordingly comprises the process involving the severalsteps and the relation and the order of one or more of such steps withrespect to each of the others and the apparatus possessing the features,properties and the relation of components which are exemplified in thefollowing detailed disclosure, and the scope of the application of whichwill be indicated in the claims.

For a fuller understanding of the nature and objects of the invention,reference should be had to the following detailed description.

In the present invention, a container for confining molten aluminum atelevated temperatures is provided by furnishing a refractory base whichis in the form of a crucible or other structure suitable for confining asubstantial pool of molten aluminum. This base is preferably formed ofcarbon or a refractory oxide such as magnesia, alumina or zirconia, or arefractory silicate such as zircon and the like. The base preferablydoes not contain any substantial quantities of materials which willoutgas as the base is heated in a vacuum chamber to the elevatedtemperature of 1200" C. and above, which is necessary for aluminumevaporation. For convenience, the invention will be initially describedin connection with the utilization of a carbon crucible withoutattempting thereby to limit the invention.

A carbon crucible, which may be in the form of a cylindrical bucket orlong boat, is formed as a solid piece or built up out of smaller piecesof carbon suitably attached together by carbon pins or the like. Thewhole interior surface of the crucible, which is to be exposed to moltenaluminum or aluminum vapors while the surface is at an elevatedtemperature, is then coated with a slurry of boron nitride to athickness on the order of inch. This slurry is then dried, such as bybaking in an oven, at a temperature on the order of 200 F. to drive offthe water or other medium for forming the liquid phase in the slurry.The thus prepared crucible is positioned in a vacuum chamber andsuitably supported so as to be heated by an induction coil, for example.Such a crucible has an extremely long life and is capable of operationat elevated temperatures in contact with molten aluminum for many hours.

While the exact reason is not fully understood, it has also been foundthat the addition of titanium, zirconium, hafnium, vanadium, niobium ortantalum to the molten aluminum tends to maintain an appreciably higherevaporation rate. This is believed to be the result of minimizing anyappreciable concentration of aluminum carbide in the melt due to pinhole porosity of the boron nitride coating which would otherwise permitpenetration of. the coating and attack of the crucible by the alummum.

After a period of operation of the crucible, it was found that the boronnitride layer has been converted to an extremely hard, dense compoundwhose identity has not been clearly established. This is believed to bea reaction product between boron nitride and aluminum, and it has beenfound to form in the vapor state as well as in the liquid state, thatis, by reaction of boron nitride with aluminum vapors as well as thereaction of boron nitride with molten aluminum.

This surface can also be formed, for example, when a cover is desiredover the crucible to provide lateral or downward evaporation from thecrucible. 'In this case, a deflecting top is provided over the crucible,a suitable opening being left between one portion of the cover and themain body of the crucible. This cover is also preferably formed of arefractory material, such as carbon, similar to the material ofthecrucible. It is equally coated with a layer of boron nitride anddried. The abutting surfaces of the crucible and cover are coated with aboron nitride layer. The crucible and cover are placed in the vacuumsystem, the crucible being charged with aluminum. The crucible and coverare then heated to about 1200 to 1300 C. and the boron nitride coatingon the cover is reacted with aluminum vapors coming from the pool ofaluminum confined by the source. These aluminum vapors react with theboron nitride coating at the elevated temperature of the cover. After afew minutes of operation, the aluminum vapors have reacted with theboron nitride coating to form a dense, hard surface which appears to besubstantially impervious to and unreactive with aluminum vapors for longperiods of time thereafter. The joint between the cover and the cruciblehas been converted to a hard, dense mass which is tight to liquidaluminum as well as to the aluminum vapors.

As a result of the techniques described above, it is possible to producesources for molten aluminum having a wide range of geometricconfigurations and a control of aluminum vapors which permits upward,sideways or downward direction of the vapors. The above described methodof forming a vapor-tight joint can also be used to form joints whichalso withstand molten aluminum, such as when a large source is to bebuilt from a number of pieces of carbon.

In one preferred embodiment of the invention, a slurry is prepared bymixing 55 grams of powdered boron nitride in 121 cc. of Water. Thismakes a paste having the consistency of whipped cream. This paste isthen applied, such as by a brush, to all those surfaces of a carboncrucible which are to be exposed to molten aluminum or aluminum vapors.This coating is preferably inch thick. The thus coated carbon body isair dried at about 200 F. Several coatings can be applied in sequence.In one preferred embodiment of the invention, a carbon crucible, havingan internal diameter of 4 and a depth of 2 inches, is thus treated withboron nitride paste and air dried. The crucible is placed in a vacuumcoating tank and is charged with 550 grams of aluminum. The crucible isthen brought up to elevated temperatures on the order of 1200 to 1300 C.At the end of 35 minutes the major percentage of the aluminum has beenevaporated. The tank is opened after the crucible is cooled to about 800C. Another 250 grams of the solid aluminum is added to the remainingmolten aluminum in the crucible. grams of titanium are also added to themelt at this time. The chamber is evacuated again and the crucible isbrought up to operating temperature. The aluminum evaporation rate inthe second run is found to be equal to or greater than the aluminumevaporation rate in the first run, the effect of the titanium additionbeing to remove any small amounts of aluminum carbide formed bypenetration of aluminum through pinholes or cracks in the boron nitridecoating.

In another embodiment of the invention a carbon crucible having aninternal diameter of 2 inches and a depth of 2 /2 inches is providedwith a /2 inch hole near the top thereof. This crucible is also providedwith a close fitting carbon cover. The inner surfaces of the crucibleand cover (as well as the mating surfaces of the crucible and cover) arethen coated with boron nitride and heated as above. This provides asource which produces a concentrated stream of aluminum vaporstravelling laterally from the hole near the top. If the crucible istilted somewhat the stream of vapors can be directed downwardly as wellas laterally. This arrangement is particularly suited for coatingdiscrete objects such as nuts and bolts or powders which are mostconveniently coated from above while being supported on a vibrating trayor the like.

While several preferred embodiments of the invention have been describedabove, numerous modifications thereof can be employed without departingfrom the spirit of the invention. The slurry of boron nitride can beprepared using numerous vehicles other than water. However, from thestandpoint of cost and freedom from residual material which might outgasin the vacuum system, water is preferred. Equally wetting agents orbinding agents can be added to the boron nitride, but these have beenfound to be unnecessary. For complex structures they can be helpful.

Similarly, carbon is a preferred material from the standpoint ofstructural strength at elevated temperatures and freedom fromdecomposition at elevated temperatures. Other refractory substances canbe employed; for example, refractory oxides such as magnesia, aluminaand zirconia or refractory silicates such as zircon can be protected bythe application of a boron nitride coating. While metals can be given atemporary coating with boron nitride, the high solubility of all metalsin molten aluminum precludes their use since any pinhole or crack in theboron nitride coating causes rapid failure of any of the metals.

While specific forms of apparatus have not been illustrated, theimproved aluminum vapor source can he employed in many types of coatingdevices such as those shown in the following U.S. Patents: 2,622,041,2,643,201, and 2,879,739 and the copending application of Cerych,Clough, and Steeves, Serial No. 795,424, filed February 25, 1959, tomention only a few of its uses.

Since certain changes can be made in the above process and apparatuswithout departing from the scope of the invention herein involved, it isintended that all matter contained in the above description shall beinterpreted as illustrative and not in a limiting sense.

What is claimed is:

1. The process of forming a container for confining molten aluminum atelevated temperatures on the order of 1200 C. and above, which containeris also resistant to attack of aluminum vapors at said elevatedtemperatures, the process comprising the steps of forming a slurrycontaining boron nitride powder, applying a layer of the boron nitrideslurry to those surfaces of a container formed of a refractory materialselected from the group consisting of carbon and the refractory oxidesand silicates which are to be exposed to aluminum at said elevatedtemperatures, drying said boron nitride layer and thereafter reactingsaid boron nitride layer with aluminum at a temperature on the order of1000 C. to 1300 C. to form a hard solid reaction product of said boronnitride and aluminum, said reaction product being unreactive withaluminum and being wettable by aluminum.

2. The process of preventing erosion of a carbon body by molten aluminumwhich comprises the steps of forming a slurry containing boron nitridepowder, applying a layer of the boron nitride slurry to the surface ofsaid carbon body, drying said layer and reacting said boron nitridelayer on the carbon body surface with aluminum vapors at a temperatureon the order of 1000 C. to 1300 C. to form a reaction product which isunreactive with aluminum and wettable by aluminum.

References Cited in the file of this patent UNITED STATES PATENTS2,185,772 McMullen Jan. 2, 1940 2,201,049 Moore May 14, 1940 2,480,299Klinke Aug. 30, 1949 2,512,230 Greaves et al. June 20, 1950 2,665,223Clough et al. Jan. 5, 1954 2,670,311 Norman et al. Feb. 23, 19542,693,521 Alexander Nov. 2, 1954 2,703,334 Clough Mar. 1, 1955 2,724,663Bond Nov. 22, 1955 2,731,366 Weinrich Jan. 17, 1956 2,768,088 Doat Oct.23, 1956 2,909,451 Lawler et al. Oct. 20, 1959 2,945,934 Kralowetz July19, 1960 2,946,699 Sanz July 26, 1960 2,996,412 Alexander Aug. 15, 1961FOREIGN PATENTS 731,990 Great Britain June 15, 1955

1. THE PROCESS OF FORMING A CONTAINER FOR CONFINING MOLTEN ALUMINIUM AT ELEVATED TEMPERATURES ON THE ORDER OF 1200* C. AND ABOVE, WHICH CONTAINER IS ALSO RESISTANT TO ATTACK OF ALUMINIUM VAPORS AT SAID ELEVATED TEMPERATURES, THE PROCESS COMPRISING THE STEPS OF FORMING A SLURRY CONTAINING BORON NITRIDE POWDER, APPLYING A LAYER OF THE BORON NITRIDE SLURRY TO THOSE SURFACES OF A CONTAINER FORMED OF A REFRACTORY MATERIAL SELECTED FROM THE GROUP CONSISTING OF CARBON AND THE REFRACTORY OXIDES AND SILICATES WHICH ARE TO BE EXPOSED TO ALUMINIUM AT SAID ELEVATED TEMPERATURES, DRYING SAID BORON NITRIDE LAYER AND THEREAFTER REACTING SAID BORON NITRIDE LAYER WITH ALUMINIUM AT A TEMPERATURE ON THE ORDER OF 1000* C. TO 1300* C. TO FORM A HARD SOLID REACTION PRODUCT BEING UNREACTIVE AND ALUMINUM, SAID REACTION PRODUCT BEING UNREACTIVE WITH ALUMINUM AND BEING WETTABLY BY ALUMINIUM. 